1. Field of the Invention
The present invention relates in general to a steering system of a motor vehicle, and more particularly to a full hydraulic power steering system for use in a forklift truck or the like, which does not require a steering gear nor a mechanical linkage
2. Description of the Prior Art
Japanese Patent First Provisional Publication No. 60-261779 shows one conventional full hydraulic power steering system which is practically applied to a forklift truck. In this system, a hydraulically operated cylinder is used for turning a steered rear wheel of the vehicle with increased power in response to turning of a steering wheel, and a computer-installed control unit is employed for compensating the position of the steering wheel under steering operation of the vehicle in accordance with a change in the relationship between the directional position assumed by the steering wheel and that assumed by the steered rear wheel. That is, the control unit calculates a stroke difference ".delta." between an actual cylinder stroke value "Xm" and a target cylinder stroke value "Xn", and when the stroke difference ".delta." exceeds a predetermined value ".delta.n", the control unit opens a fluid drain line through which a part of the fluid in the cylinder operating line is drained into a fluid drain tank. With this, the steering wheel becomes idled but temporally thereby carrying out position compensation or correction of the steering wheel automatically.
However, in this conventional system, the position compensation of the steering wheel is carried out irrespective of the turning speed of the steering wheel. Thus, even when the steering wheel is slowly turned by an operator, the idling opration of the steering wheel takes place upon the stroke difference ".delta." exceeding the predetermined value ".delta.n". This induces however such a drawback that under slow turning of the steering wheel, the travelling direction of the vehicle will not change even when the steering wheel is being turned. This phenomenon makes the operator uncomfortable and uneasy.